Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse x/y movement by using two defined axes which contain a collection of sensor elements that detect the position of a conductive object, such as a finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.
One type of touchpad operates by way of capacitance sensing utilizing capacitive sensors. The capacitance detected by a capacitive sensor changes as a function of the proximity of a conductive object to the sensor. The conductive object can be, for example, a stylus or a user's finger. In a touch-sensor device, a change in capacitance detected by each sensor in the X and Y dimensions of the sensor array due to the proximity or movement of a conductive object can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitive sensor is processed by a processing device, which in turn produces electrical or optical signals representative of the position of the conductive object in relation to the touch-sensor pad in the X and Y dimensions. A touch-sensor strip, slider, or button operates on the same capacitance-sensing principle.
Another user interface device that has become more common is a touch screen. Touch screens, also known as touchscreens, touch panels, or touchscreen panels are display overlays which are typically either pressure-sensitive (resistive), electrically-sensitive (capacitive), acoustically-sensitive (SAW—surface acoustic wave) or photo-sensitive (infra-red). The effect of such overlays allows a display to be used as an input device, removing the keyboard and/or the mouse as the primary input device for interacting with the display's content. Such displays can be attached to computers or, as terminals, to networks. There are a number of types of touch screen technologies, such as optical imaging, resistive, surface acoustical wave, capacitive, infrared, dispersive signal, piezoelectric, and strain gauge technologies. Touch screens have become familiar in retail settings, on point of sale systems, on ATMs, on mobile handsets, on kiosks, on game consoles, and on PDAs where a stylus is sometimes used to manipulate the graphical user interface (GUI) and to enter data.
FIG. 1A illustrates a conventional touch-sensor pad. The touch-sensor pad 100 includes a sensing surface 101 on which a conductive object may be used to position a pointer in the x- and y-axes, using either relative or absolute positioning, or to select an item on a display. Touch-sensor pad 100 may also include two buttons, left and right buttons 102 and 103, respectively, shown here as an example. These buttons are typically mechanical buttons, and operate much like a left and right buttons on a mouse. These buttons permit a user to select items on a display or send other commands to the computing device.
FIG. 1B illustrates a conventional linear touch-sensor slider. The linear touch-sensor slider 110 includes a surface area 111 on which a conductive object may be used to control a setting on a device, such as volume or brightness. Alternatively, the linear touch-sensor slider 110 may be used for scrolling functions. The construct of touch-sensor slider 110 may be the same as that of touch-sensor pad 100. Touch-sensor slider 110 may include a sensor array capable of detection in only one dimension (referred to herein as one-dimensional sensor array). The slider structure may include one or more sensor elements that may be conductive traces. By positioning or manipulating a conductive object in contact or in proximity to a particular portion of the slider structure, the capacitance between each conductive trace and ground varies and can be detected. The capacitance variation may be sent as a signal on the conductive trace to a processing device. It should also be noted that the sensing may be performed in a differential fashion, obviating the need for a ground reference. For example, by detecting the relative capacitance of each sensor element, the position and/or motion (if any) of the external conductive object can be determined. It can be determined which sensor element has detected the presence of the conductive object, and it can also be determined the motion and/or the position of the conductive object over multiple sensor elements.
One difference between touch-sensor sliders and touch-sensor pads may be how the signals are processed after detecting the conductive objects. Another difference is that the touch-sensor slider is not necessarily used to convey absolute positional information of a conducting object (e.g., to emulate a mouse in controlling pointer positioning on a display), but rather relative positional information. However, the touch-sensor slider and touch-sensor pad may be configured to support either relative or absolute coordinates, and/or to support one or more touch-sensor button functions of the sensing device.
FIG. 1C illustrates a conventional sensing device having three touch-sensor buttons. Conventional sensing device 120 includes button 121, button 122, and button 123. These buttons may be capacitive touch-sensor buttons. These three buttons may be used for user input using a conductive object, such as a finger.
In order to detect the presence of a conductive object on either of the above mentioned sensing devices (e.g., touch-sensor pad 100, touch-sensor slider 110, or touch-sensor buttons of sensing device 120), a current source is coupled to the sensing device to provide a charge current to one or more sensor elements of the sensing device. The current source may be part of a relaxation oscillator. The output of the relaxation oscillator may be measured by a digital counter.
One conventional sensing device includes a current source that provides current to the sensing elements of the sensing device to measure the capacitance on the sensing elements. The conventional sensing device, however, has a fixed value for the current. This fixed value may be a hard coded value in a register programmable current output digital-to-analog converter (DAC) (also known as IDAC). The hard coded value may be stored in a register or in memory of the processing device, which is used to determine the presence and/or position of a conductive object on the sensing device.
Conventional sensing devices that use a hard coded current from the current source of the capacitive circuit can have current or circuit response variations due to chip, system, and/or board manufacturing variations. These current or circuit response variations may result in improper capacitive sensing operations. For example, if the current is too low, it may take the circuit longer to measure the capacitance on the sensing device. Current or circuit response variations may also result in significant failure rate in production quantities due to the manufacturing variations.
FIG. 1D illustrates a varying capacitance sensor element. In its basic form, a capacitance sensor element 130 is a pair of adjacent conductors 131 and 132. There is a small edge-to-edge capacitance, but the intent of sensor element layout is to minimize the parasitic capacitance CP between these conductors. When a conductive object 133 (e.g., finger) is placed in proximity to the two conductors 131 and 132, there is a capacitance between electrode 131 and the conductive object 133 and a similar capacitance between the conductive object 133 and the other electrode 132. The capacitance between the electrodes when no conductive object 133 is present is the base capacitance CP that may be stored as a baseline value. There is also a total capacitance (CP+CF) on the sensor element 130 when the conductive object 133 is present on or in close proximity to the sensor element 130. The baseline capacitance value CP may be subtracted from the total capacitance when the conductive object 133 is present to determine the change in capacitance (e.g., capacitance variation CF) when the conductive object 133 is present and when the conductive object 133 is not present on the sensor element. Effectively, the capacitance variation CF can be measured to determine whether a conductive object 133 is present or not (e.g., sensor activation) on the sensor element 130.
Capacitance sensor element 130 may be used in a capacitance sensor array. The capacitance sensor array is a set of capacitors where one side of each capacitor is connected to a system ground 138. When the capacitance sensor element 130 is used in the sensor array, when the conductor 131 is sensed, the conductor 132 is connected to ground, and when the conductor 132 is sensed, the conductor 131 is connected to ground. Alternatively, when the sensor element is used for a touch-sensor button, the sensor element is sensed and the sensed button area is surrounded by a fixed ground. The presence of the conductive object 133 increases the capacitance (CP+CF) of the sensor element 130 to ground. Determining sensor element activation is then a matter of measuring change in the capacitance (CF) or capacitance variation. Sensor element 130 is also known as a grounded variable capacitor.
The conductive object 133 of FIG. 1D has been illustrated as a finger. Alternatively, this technique may be applied to any conductive object, for example, a conductive door switch, position sensor, or conductive pen in a stylus tracking system (e.g., stylus).
The capacitance sensor element 130 is known as a projected capacitance sensor. Alternatively, the capacitance sensor element 130 may be a surface capacitance sensor that does not make use of rows or columns, but instead makes use of a single linearized field, such as the surface capacitance sensor described in U.S. Pat. No. 4,293,734. The surface capacitance sensor may be used in touch screen applications.
FIG. 1E illustrates a conventional capacitance sensor element 137 coupled to a processing device 110. Capacitance sensor element 137 illustrates the capacitance as seen by the processing device 110 on the capacitance sensing pin 136. As described above, when a conductive object 133 (e.g., finger) is placed in proximity to one of the conductors 135, there is a capacitance, CF, between the one of the conductors 135 and the conductive object 133 with respect to ground. This ground, however, may be a floating ground. Also, there is a capacitance, CP, between the conductors 135, with one of the conductors 135 being connected to a system ground 138. The grounded conductor may be coupled to the processing device 110. The conductors 135 may be metal, or alternatively, the conductors may be conductive ink (e.g., carbon ink), conductive ceramic (e.g., transparent conductors of indium tin oxide (ITO)), or conductive polymers. The grounded conductor may be an adjacent sensor element. Alternatively, the grounded conductor may be other grounding mechanisms, such as a surrounding ground plane. Accordingly, the processing device 110 can measure the change in capacitance, capacitance variation CF, as the conductive object is in proximity to one of the conductors 135. Above and below the conductor that is closest to the conductive object 133 is dielectric material 134. The dielectric material 134 above the conductor 135 can be an overlay, as described in more detail below. The overlay may be non-conductive material used to protect the circuitry from environmental conditions and ESD, and to insulate the user's finger (e.g., conductive object) from the circuitry. Capacitance sensor element 137 may be a sensor element of a touch-sensor pad, a touch-sensor slider, or a touch-sensor button.
One conventional circuit of measuring the change in capacitance introduced by the conductive object is a relaxation oscillator.
FIG. 1F illustrates a conventional relaxation oscillator for measuring capacitance on a sensor element. The relaxation oscillator 150 is formed by the capacitance to be measured on the sensor element, represented as capacitor 151, a charging current source 152, a comparator 153, and a reset switch 154 (also referred to as a discharge switch). It should be noted that capacitor 151 is representative of the capacitance measured on a sensor element of a sensor array. The relaxation oscillator is coupled to drive a charging current (Ic) 157 in a single direction onto a device under test (“DUT”) capacitor, capacitor 151. As the charging current piles charge onto the capacitor 151, the voltage across the capacitor increases with time as a function of Ic 157 and its capacitance C. Equation (1) describes the relation between current, capacitance, voltage, and time for a charging capacitor.CdV=ICdt   (1)
The relaxation oscillator begins by charging the capacitor 151, at a fixed current Ic 157, from a ground potential or zero voltage until the voltage across the capacitor 151 at node 155 reaches a reference voltage or threshold voltage, VTH 160. At the threshold voltage VTH 160, the relaxation oscillator allows the accumulated charge at node 155 to discharge (e.g., the capacitor 151 to “relax” back to the ground potential) and then the process repeats itself. In particular, the output of comparator 153 asserts a clock signal FOUT 156 (e.g., FOUT 156 goes high), which enables the reset switch 154. This discharges the capacitor at node 155 to ground and the charge cycle starts again. The relaxation oscillator outputs a relaxation oscillator clock signal (FOUT 156) having a frequency (fRO) dependent upon capacitance C of the capacitor 151 and charging current Ic 157.
The comparator trip time of the comparator 153 and reset switch 154 add a fixed delay. The output of the comparator 153 is synchronized with a reference system clock to guarantee that the reset time is long enough to completely discharge capacitor 151. This sets a practical upper limit to the operating frequency. For example, if capacitance C of the capacitor 151 changes, then fRO changes proportionally according to Equation (1). By comparing fRO of FOUT 156 against the frequency (fREF) of a known reference system clock signal (REF CLK), the change in capacitance ΔC can be measured. Accordingly, equations (2) and (3) below describe that a change in frequency between FOUT 156 and REF CLK is proportional to a change in capacitance of the capacitor 151.ΔC ∝Δf, where   (2)Δf=fRO−fREF.   (3)
A frequency comparator may be coupled to receive relaxation oscillator clock signal (FOUT 156) and REF CLK, compare their frequencies fRO and fREF, respectively, and output a signal indicative of the difference Δf between these frequencies. By monitoring Δf one can determine whether the capacitance of the capacitor 151 has changed.
The relaxation oscillator 150 may be built using a programmable timer (e.g., 555 timer) to implement the comparator 153 and reset switch 154. Alternatively, the relaxation oscillator 150 may be built using other circuitry. The capacitor charging current for the relaxation oscillator 150 may be generated in a register programmable current output DAC (also known as IDAC). Accordingly, the current source 152 may be a current DAC or IDAC. The IDAC output current may be set by an 8-bit value provided by the processing device 110, such as from the processing core. The 8-bit value may be stored in a register or in memory.
In many capacitance sensor element designs, the two “conductors” (e.g., 131 and 132) of the sensing capacitor are actually adjacent sensor elements that are electrically isolated (e.g., PCB pads or traces), as indicated in FIG. 1D. Typically, one of these conductors is connected to a system ground 138. Layouts for touch-sensor slider (e.g., linear slide sensor elements) and touch-sensor pad applications have sensor elements that may be immediately adjacent. In these cases, all of the sensor elements that are not active are connected to a system ground 138 of the processing device 110. The actual capacitance between adjacent conductors is small (CP), but the capacitance of the active conductor (and its PCB trace back to the processing device 110) to ground, when detecting the presence of the conductive object 133, may be considerably higher (CP+CF). The capacitance of two parallel conductors is given by the following equation:
                    C        =                                            ɛ              0                        ·                          ɛ              R                        ·                          A              d                                =                                                    ɛ                R                            ·              8.85              ·                              A                d                                      ⁢            pF            ⁢                          /                        ⁢            m                                              (        4        )            
The dimensions of equation (4) are in meters. This is a very simple model of the capacitance. The reality is that there are fringing effects that substantially increase the sensor element-to-ground (and PCB trace-to-ground) capacitance.
As described above with respect to the relaxation oscillator 150, when a finger or conductive object is placed on the sensor element, the capacitance increases from CP to CP+CF so the relaxation oscillator output signal 156 (FOUT) decreases in frequency. The relaxation oscillator output signal 156 (FOUT) may be fed to a digital counter for measurement. There are two methods for counting the relaxation oscillator output signal 156: frequency measurement and period measurement.
In the conventional relaxation oscillator, the baseline capacitance CP is with respect to system ground 138, such as of the processing device 110, while the capacitance variation CF is with respect to a common ground or a floating ground.
Sensing devices with floating grounds are subject to high voltage AC offsets due to the sensing devices power supply, especially when used with AC/DC converters with high leakage or when the sensing device is coupled to the AC line. Capacitive sensor elements may be especially sensitive to this type of periodic noise due to the low currents and capacitance of the sensing device.
FIG. 1G illustrates a graph of the voltage across a sensor element. The voltage Va 161, which is the voltage across the capacitor 151 at node 155. As the voltage Va 161 reaches a threshold voltage VTH 160, the voltage is removed from the node 155, dropping the voltage Va 161 back to ground. As described above, the conventional relaxation oscillator is subject to noise on the system ground. The system ground may be very noisy in comparison to the common ground. Consequently, the frequency of the voltage Va is modulated by the noise, resulting in different charge time periods (t1, t2, and t3) between peaks of the saw-toothed voltage. This may affect sensing the correct capacitance variation CF.
FIG. 1H illustrates two graphs of the output counts for four scans of four buttons on a conventional sensing device with and without alternating current (AC) noise. The presence of a finger or other type of conductive object on the switch may be determined by the difference in counts between a stored value (e.g., baseline or threshold) for no switch actuation and the acquired value with switch actuation. The sensing device can be scanned to measure the capacitance, which is represented by the number of counts. When the counts are measured as being above a “button pressed” or presence threshold, switch activation can be detected. Graph 160 illustrates the counts measured on a conventional sensing device that includes four buttons. Graph 160 illustrates the counts of a first scan of the four buttons (four shaded rectangles) in a row followed by a delay and then their rescan three times (four total scans of the four buttons). The result is that counts of all buttons exceed the ‘button pressed’ threshold consistently when no AC noise is present. Graph 160 illustrates the counts measured on the conventional sensing device when no or minimal AC noise is present.
Graph 170 illustrates the counts measured on the conventional sensing device when AC noise is present. Similar to Graph 160, Graph 170 illustrates the counts of a first scan of the four buttons (four shaded rectangles) in a row followed by a delay and then their rescan three times (four total scans of the four buttons). Graph 170, however, illustrates the AC noise offsets introduced in the counts measured on the conventional sensing device. The AC noise may be caused due to an AC power adapter that is used to power the sensing device, or alternatively, from other AC power sources. Due to the AC noise, the button counts measured on the conventional sensing device do not consistently cross the “button pressed” or presence threshold.
Conventional sensing devices do not attempt to reduce the AC noise level. Taking a single sample of length t, asynchronous to the AC noise, results in an offset proportional to the AC noise amplitude and polarity at the time of the sample. Because the precise frequency and phase of the AC noise is not known during the sample, the offset appears to be random to the firmware. Because conventional designs perform no reduction in AC noise, the worst case AC induced noise level results in being approximately 5 times the signal level. The characteristic waveform of this worst case (e.g., counts measured including the AC offset) may be similar to that of an object in proximity to the capacitive sensing circuit, meaning the counts may exceed the presence threshold for switch activation. The result is false-positive detections and missed-real detections of the presence of the conductive object, resulting in detection algorithm instability.